Recently, chemists have sought to synthesize oligomers for high performance advanced composites suitable for aerospace applications. These composites should exhibit solvent resistance, be strong, tough, and impact resistant; be easy to process; and be thermoplastic. Oligomers and composites that have thermooxidative stability, and, accordingly, can be used at elevated temperatures are particularly desirable.
While epoxy-based composites are suitable for many applications, their brittle nature and susceptibility to degradation make them inadequate for many aerospace applications, especially those applications which require thermally stable, tough composites. Accordingly, research has recently focused on polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness. Still the maximum temperatures for use of the polyimide composites, such as PMR-15, are about 600.degree.-625.degree. F., since they have glass transition temperatures of about 690.degree. F. PMR-15, however, suffers from brittleness.
There has been a progression of polyimide sulfone compounds synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode synthesized maleic-capped linear polyarylimides as disclosed in U.S. Pat. No. 3,839,287. Holub and Evans synthesized maleic- or nadic-capped, imido-substituted polyester compositions as disclosed in U.S. Pat. No. 3,729,446. Monacelli proposed tetra-maleimides made through an amic acid mechanism with subsequent ring closure, as shown in U.S. Pat. Nos. 4,438,280 or 4,418,181. We synthesized thermally stable polysulfone oligomers as disclosed in U.S. Pat. No. 4,476,184 or U.S. Pat. No. 4,536,559, and have continued to make advances with polyetherimidesulfones, polybenzoxazolesulfones, polybutadienesulfones, and "star" or "star-burst" multidimensional oligomers. We have shown surprisingly high glass transition temperatures yet reasonable ease of processing and desirable physical properties in many of these oligomers and their composites.
Polybenzoxazoles or their corresponding heterocycles, such as those disclosed in our U.S. Pat. No. 4,965,336 (to Luvowitz & Sheppard ) and U.S. Pat. No. 4,868,270 (to Lubowitz & Sheppard) and Stephenson), may be used at temperatures up to about 750.degree.-775.degree. F., since these composites have glass transition temperatures of about 840.degree. F. Some aerospace applications need composites which have even higher use temperatures while maintaining toughness, solvent resistance, ease of processing, formability, strength, and impact resistance.
Multidimensional oligomers, such as disclosed in our copending applications U.S. Ser. Nos. 07/167,656 and 07/176,158, and U.S. Pat. No. 5,210,313 have superior processing ease than some advanced composite oligomers since they can be handled at lower temperatures. Upon curing, however, the phenylimide end caps crosslink so that the thermal resistance and stiffness of the resulting composite is markedly increased. This increase is obtained with only a minor loss of matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties. Glass transition temperatures above 850.degree. F. are achievable.
Commercial polyesters, when combined with well-known reactive diluents, such as styrene, exhibits marginal thermal and oxidative resistance, and are useful only for aircraft or aerospace interiors. Polyarylesters are often unsatisfactory, also, since the resins often are semicrystalline which may make them insoluble in useable laminating solvents, intractable in fusion under typical processing conditions, and difficult and expensive to manufacture because of shrinking and/or warping. Those polyarylesters that are soluble in conventional laminating solvents remain so in composite form, thereby limiting their usefulness in structural composites. The high concentration of ester groups contributes to resin strength and tenacity, but also tends to make the resin susceptible to the damaging effects of water absorption. High moisture absorption by commercial polyesters can lead to lowering of the glass transition temperature leading to distortion of the composite when it is loaded at elevated temperature.
High performance, aerospace, polyester advanced composites, however, can be prepared using crosslinkable, end capped polyesterimide ethersulfone oligomers that have an acceptable combination of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. By including Schiff base (--CH.dbd.N--) linkages in the oligomer chain, the linear, advanced composites formed with polyester oligomers of our copending application U.S. Ser. No. 07/137,493 can have semiconductive or conductive properties when appropriately doped or reacted with appropriate metal salts.
Conductive and semiconductive plastics have been extensively studied (see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987, 4,344,869; and 4,344,870), but these polymers do not possess the blend of properties which are essential for aerospace applications. That is, conductive polymers do not possess the blend of (1) toughness, (2) stiffness, (3) ease of processing, (4) impact resistance (and other matrix stress transfer capabilities), (5) retention of properties over a broad range of temperatures, and (6) thermooxidative resistance that is desirable in aerospace advanced composites. The prior art composites are often too brittle.
Thermally stable multidimensional oligomers having semiconductive or conductive properties when doped with suitable dopants are also known and are described in our copending applications (including U.S. Ser. Nos. 06/773,381 and 07/212,404 to Lubowitz, Sheppard and Torre). The linear arms of the oligomers contain conductive linkages, such as Schiff base (--N.dbd.CH--) linkages, between aromatic groups. Sulfone and ether linkages are interspersed in the arms. Each arm is terminated with a mono- or difunctional end cap (i.e. an end cap having one or two crosslinking functionalities) to allow controlled crosslinking upon heat-induced or chemically-induced curing. Other "semiconductive" oligomers are described in our other copending applications.
Polyamide oligomers and blends are described in U.S. Pat. Nos. 4,935,523; 4,847,333; and 4,876,328 and polyetherimide oligomers and blends are described in U.S. Pat. No. 4,851,495.
Polyamideimides are generally injection-moldable, amorphous, engineering thermoplastics which absorb water (swell) when subjected to humid environments or when immersed in water. Polyamideimides are generally described in the following patents: U.S. Pat. Nos. 3,658,938; 4,628,079; 4,599,383; 4,574,144; or 3,988,374. The thermal integrity and solvent-resistance can be greatly enhanced by capping amideimide backbones with monomers that present one or two crosslinking functionalities at each end of the oligomer, as will be described.
Advanced composite blends, as we use that term, contain a blend of at least one oligomer from one chemical family and at least one polymer from a different chemical family. These advanced composite blends yield composites that possess properties intermediate to the properties of composites made from either pure component. For example, a polybenzoxazole oligomer can be blended with a polyethersulfone polymer to improve the flexibility (reduce the stiffness) of the resulting composite without significant reduction of the other, desired, high performance properties of the heterocycle (i.e. oxazole). We described these advanced composite blends in U.S. Ser. No. 07/167,604, abandoned in favor of U.S. Ser. No. 07/619,677, filed Nov. 29, 1990.
A major problem encountered in improving high temperature mechanical and physical properties of reinforced resin, composites occurs due to inadequate transfer of induced matrix stress to the reinforcement. The matrix also helps to prevent the fiber from buckling. Sizing is often applied to the reinforcing fibers to protect the fibers during processing and to enhance bonding at this interface between the fibers and the resin matrix thereby more efficiently transferring the load and stabilizing the fiber. Sizings are essentially very thin films of resin (less than a few wt %) applied to the fibers. To be effective, they should be relatively high MW materials that form a relatively uniform coating. Commercially available sizings include epoxy sizings under the trade designations UC309 and UC314 from Amoco, G or W from Hercules, EP03 from Hoechst and high performance sizings under the trade designations L30N, L20N, UC0121 or UC0018 from Amoco. Commercially available sizings are unsatisfactory because they are generally monomers or low MW materials that often only partially coat the fibers and, as a result, minimally improve composite properties. There is a need, therefore, for improved sizings, especially for carbon fibers intended for high performance composites.